This invention relates generally to techniques for generating small-diameter, highly focused optical beams. There are several applications using focused beams where the attainable beam diameter is limited by diffraction. Examples of such applications are: (1) lithography processes as used in the fabrication of semiconductor circuits; (2) microscopy systems in which a specimen must be scanned by the smallest possible beam spot; (3) optical disc media in which recording density is dependent on the spot size of a recording beam; and (4) directed energy beam systems, whether used as weapons or as industrial metal cutting tools, where a highly focused beam is desired.
In these and other similar applications, diffraction effects limit the degree to which a beam can be focused to an extremely small spot size. For example, in high resolution optical lithography as used in the fabrication of semiconductor chips it is well known that a significant limitation is the diameter of an optical beam (or particle beam) impinging on a work surface. More specifically, in a lithography technique known as maskless lithography, often referred to as ML2, various approaches have been proposed but basically they have in common that instead of a lithography mask, one or multiple optical (or particle) beams are focused onto a semiconductor surface to perform a desired fabrication lithography function. Regardless of whether one beam or multiple beams are used, and regardless of whether the beam is an optical beam or a particle beam, resolution is limited by the beam diameter and conventional optical focusing techniques to reduce the beam diameter are ultimately limited by diffraction effects. Using radiation of smaller wavelengths extends the resolution of lithography systems but also has limitations. The present invention provides a novel approach for focusing an optical beam and thereby extending the overall resolution of the system without the need to use smaller wavelengths.